Method of purifying aromatic polycarboxylic acid

ABSTRACT

Crude polycarboxylic acid is slurried in an aqueous medium and the slurry is brought into contact with a metal catalyst while preventing catalyst components thereof from contaminating crystals. Since hydrogenation or decarbonylation of a polymerization inhibitory substance or a substance causing coloration can efficiently proceed at a low temperature while suppressing side reactions, a product having such a quality as to permit direct use thereof as such for polymerization can be obtained with good productivity. Also, since the contact temperature can be lowered, simplification of apparatus and energy saving may be attained.

TECHNICAL FIELD

The present invention relates to a method of purifying a crude aromaticpolycarboxylic acid and, more specifically, to a purification method inwhich a crude aromatic polycarboxylic acid obtained by liquid phaseoxidation of a polyalkyl aromatic hydrocarbon is purified by the removalof polymerization inhibitory substances and substances causingcoloration therefrom to give a purified aromatic polycarboxylic acidwhich can be used directly as such for the polymerization resulting in ahigh molecular weight, colorless polyester resin, etc.

BACKGROUND ART

Aromatic polycarboxylic acids are commercially important substances aschemical intermediates. Thus, there is a wide demand for aromaticpolycarboxylic acids as raw materials of polyesters, polyamides,polyimides, liquid crystal polymers, etc. which are used for fibers,bottles, films and electronic applications.

As currently widely industrially used aromatic polycarboxylic acids,there may be mentioned terephthalic acid, isophthalic acid, phthalicacid, trimellitic acid, pyromellitic acid, 2,6-naphthalenedicarboxylicacid, 4,4′-biphenyldicarboxylic acid, 1,4,5,8-naphthalenetetracarboxylicacid and 3,3′,4,4′-biphenyltetracarboxylic acid.

Known methods for the preparation of an aromatic polycarboxylic acidinclude a method in which a polyalkyl aromatic hydrocarbon such asxylene, dialkylnaphthalene, dialkylbiphenyl, tetraalkylnaphthalene ortetraalkylbiphenyl is oxidized with molecular oxygen at a hightemperature and a high pressure in the presence of a heavy metal such asCo or Mn and a bromine compound in an acetic acid solvent, and a methodin which the polyalkyl aromatic hydrocarbon is oxidized with air in thepresence of nitric acid, chromic acid or the like. The aromaticpolycarboxylic acid obtained by the above oxidation reaction containsimpurities such as monocarboxylic acids and aldehydes which areintermediate products of the oxidation reaction, bromine adducts andmetal components which are derived from the catalyst and coloringsubstances having unknown structures.

As a recent increase of necessity for recycling plastic materials suchas polyesters, materials are now recycled and reused through, forexample, decomposition of PET bottles. In general, however, aromaticpolycarboxylic acids obtained by the above decomposition containimpurities such as colored substances and foreign matters.

When the aromatic polycarboxylic acids containing such impurities areused as raw materials for the polymerization with diols or diamines,physical and mechanical properties, such as heat resistance, mechanicalstrengths and dimensional stability, of the obtained resins areinferior. Therefore, such aromatic polycarboxylic acids cannot be usedas raw materials for polyesters, polyamides and polyimides. Further,crude aromatic polycarboxylic acids obtained by oxidation are generallycolored yellow or black and cannot be used as such for applicationsrequiring transparency such as bottles and films.

In this circumstance, as a method of purifying terephthalic acid, forexample, a method is widely used in which a crude terephthalic acid iscompletely dissolved in water as a solvent at a high temperature of 260to 280° C. The solution is then subjected to hydrogenation using apalladium catalyst supported on activated carbon so that impurities suchas polymerization inhibitory substances and substances causingcoloration are reduced. From the resulting solution, terephthalic acidis crystallized. By this method, purified terephthalic acid capable ofbeing directly used as such for polymerization may be obtained (JapanesePatent Publication No. 41-16860).

The above method is for terephthalic acid which is easily soluble inwater at a high temperature. In order to improve productivity, however,it is necessary to use a temperature as high as 260 to 280° C. and,accordingly, to use a high pressure. Because such a high temperature isused, side reactions such as hydrogenation on the nucleus are apt tooccur and, further, it is necessary to select materials of the apparatuswhile taking corrosion thereof into consideration.

In the case of naphthalenedicarboxylic acid and biphenyldicarboxylicacid, since the solubility thereof in water is about 1/10 of that ofterephthalic acid, it is necessary to use much higher temperature than280° C. in order to conduct the above purification method with highproductivity. This causes extreme difficulty in practical use.

Purification of an organic compound is generally performed bydistillation, crystallization, adsorption or a combination of theseoperations. Since aromatic polycarboxylic acids have aself-decomposition temperature which is lower than the boiling pointthereof, the purification by distillation is substantially impossible.Further, since aromatic polycarboxylic acids have poor solubility incommonly industrially used solvents, the purification by crystallizationinvolves difficulties. In particular, since naphthalenepolycarboxylicacid and biphenylpolycarboxylic acid are hardly soluble in varioussolvents, industrially advantageous processes for producing high puritynaphthalenepolycarboxylic acid or high purity biphenylpolycarboxylicacid have not yet been established.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a method ofpurifying an aromatic polycarboxylic acid capable of efficientlypurifying aromatic polycarboxylic acid, which is difficult to bepurified as described above, to give the aromatic polycarboxylic acidwhich can be used directly as such for the polymerization resulting in ahigh molecular weight, colorless polyester resin, etc.

The inventors have made an earnest study on a method of purifying anaromatic polycarboxylic acid which method has such problems as describedabove. As a result, it has been found that when the aromaticpolycarboxylic acid is contacted with a metal catalyst in the absence ofoxygen, while maintaining the aromatic polycarboxylic acid in a slurriedstate, i.e. at such a temperature that the aromatic polycarboxylic aciddissolved in an aqueous medium exists together with undissolved aromaticpolycarboxylic acid, and while preventing catalytic components fromcontaminating the purified aromatic polycarboxylic acid, impurities suchas intermediate products of the oxidation reaction and coloringsubstances can be removed by hydrogenation or decarbonylation at a lowtemperature while preventing the production of by-products so that thepurification can be achieved with a good productivity to give aromaticpolycarboxylic acid which can be used directly as such for thepolymerization resulting in a high molecular weight, colorless polyesterresin, etc. The present invention has been made on the basis of theabove finding.

Thus, the present invention provides a method of purifying an aromaticpolycarboxylic acid, comprising a step of slurrying a crude aromaticpolycarboxylic acid in an aqueous medium and a step of bringing theslurry into contact with a metal catalyst in the absence of oxygen whilepreventing catalyst components from contaminating crystals.

BEST MODE FOR CARRYING OUT THE INVENTION

The aromatic polycarboxylic acid used for the purpose of the presentinvention is an aromatic hydrocarbon, such as benzene, naphthalene orbiphenyl, to which two or more carboxyl groups are linked. A method ofproducing such an aromatic polycarboxylic acid is not specificallylimited. For example, the aromatic polycarboxylic acid may be obtainedby oxidizing a raw material compound obtained by introducing an alkylgroup such as a methyl group, an ethyl group or an isopropyl group and aplurality of functional groups capable of forming carboxyl groups byoxidation, such as formyl groups and acetyl groups, into theabove-mentioned aromatic hydrocarbon.

As aromatic polycarboxylic acids which are currently industrially widelyused, there may be mentioned terephthalic acid, isophthalic acid,phthalic acid, trimellitic acid, pyromellitic acid,2,6-naphthalenedicarboxylic acid, 4,4′-biphenyldicarboxylic acid,1,4,5,8-naphthalenetetracarboxylic acid and3,3′,4,4′-biphenyltetracarboxylic acid.

Formylated compounds produced as intermediate compounds during thecourse of the production of aromatic polycarboxylic acid by oxidation ofan aromatic hydrocarbon having a plurality of substituents, such as4-carboxybenzaldehyde in the case of the production of terephthalic acidand formylnaphthoic acid in the case of the production ofnaphthalenedicarboxylic acid, are impurities which are difficult to beremoved and which act as polymerization inhibitory substances andsubstances causing coloration in the subsequent polymerization stage.

In the method of the present invention, the above-describepolymerization inhibitory substances and substances causing colorationcontained in a crude aromatic polycarboxylic acid are hydrogenated ordecarbonylated by contact with a metal catalyst in the absence ofoxygen. When the contact with the metal catalyst is carried out in thepresence of hydrogen, the polymerization inhibitory substances andsubstances causing coloration are hydrogenated. When the contact withthe metal catalyst is carried out in the absence of hydrogen and oxygen,the polymerization inhibitory substances and substances causingcoloration are decarbonylated. By this way, the impurities are removed.

Any metal catalyst may be used as the hydrogenation or decarbonylationcatalyst as long as it has an activity and is hardly deactivated in thepurification conditions. A catalyst having a carrier on which catalyticcomponents are supported is generally used.

As metals to be supported, there may be mentioned Group 8 metals, namelynoble metals such as platinum, palladium, ruthenium, rhodium, osmium andiridium, cobalt and nickel. As the carrier, activated carbon ispreferably used for reasons of resistance to aromatic carboxylicacid-containing high temperature aqueous solution.

The temperature and pressure at which the hydrogenation ordecarbonylation is performed vary with the kind of the aromaticpolycarboxylic acid to be purified, the conditions of the impurities andthe catalyst used and are selected so that the hydrogenation ordecarbonylation of the polymerization inhibitory substances andsubstances causing coloration can be efficiently achieved whilepreventing occurrence of side reactions.

The present invention is characterized in that the aromaticpolycarboxylic acid is contacted with a metal catalyst while maintainingthe aromatic polycarboxylic acid in a slurried state, i.e. at such astate that a portion of the aromatic polycarboxylic acid is dissolved inan aqueous medium, and while preventing catalytic components fromcontaminating the purified aromatic polycarboxylic acid.

Thus, at the outset, the temperature at which the hydrogenation ordecarbonylation is carried out is so selected that the dissolvedaromatic polycarboxylic acid and undissolved aromatic polycarboxylicacid coexist in the form of a slurry, though the temperature varies withthe kind of the aromatic polycarboxylic acid to be purified.

For example, since the solubility of terephthalic acid in water at 230°C. is 6.5 g/100 g, a slurry in which dissolved terephthalic acid andundissolved terephthalic acid coexist is formed when the amount ofterephthalic acid relative to water is beyond the solubility.

Since the hydrogenation or decarbonylation is carried out for a slurrycontaining undissolved aromatic polycarboxylic acid rather than for ahomogeneous aqueous solution, it is necessary to contrive a way forusing the catalyst.

Namely, it is necessary to retain the metal catalyst so that thecatalyst and the undissolved aromatic polycarboxylic acid are preventedfrom being mixed together. For example, the catalyst particles can beprevented from being mixed into the purified aromatic polycarboxylicacid by holding the catalyst in a basket through which only the slurryis permitted to pass and by immersing the basket in the slurry.

The concentration of the slurry is so selected that the hydrogenation ordecarbonylation of the purities is not hindered and that the slurry canbe transferred using an ordinary industrial means.

When the aromatic polycarboxylic acid is contacted with the metalcatalyst in the presence of hydrogen, the hydrogenation can be achievedby injecting hydrogen into water as a solvent in which part of thearomatic polycarboxylic acid is dissolved.

The hydrogen partial pressure in the hydrogenation is so selected thatthe hydrogenation of the aromatic nucleus of the aromatic polycarboxylicacid is prevented from occurring at the selected temperature asdescribed above but that the hydrogenation of the formylated compoundsacting as polymerization inhibitory substances and substances causingcoloration can efficiently proceed.

Namely, when hydrogenation proceeds excessively, impurities willincrease. Thus, the hydrogen partial pressure in the hydrogenation ispreferably 0.1 to 3 MPa.

When the aromatic polycarboxylic acid is contacted with the metalcatalyst in the absence of hydrogen and oxygen, it is necessary tosubstitute the atmosphere in the system with an inert gas such asnitrogen so that oxygen is completely removed.

In the present invention, the term “in the absence of oxygen” isintended to refer to the state in which the atmosphere in the system issubstituted with an inert gas such as nitrogen so that oxygen completelydisappears in the system, i.e. in which the oxygen content is 1 ppm orless, preferably 0.1 ppm or less. As the inert gas to be used for thispurpose is most generally nitrogen. Argon may be used. Carbon dioxide isnot preferable.

The residence time varies with the kind of the aromatic polycarboxylicacid to be purified and the state of the impurities but is so selectedthat the hydrogenation or decarbonylation can be nearly completed. Ingeneral, the residence time is 0.5 to 5 h.

Generally, when the hydrogenation or decarbonylation is nearlycompleted, the mixture is cooled to near room temperature. The crystalsthus obtained are rinsed with warm water, etc. and then dried to obtaina purified aromatic polycarboxylic acid.

According to the present invention, since a large amount of waterrequired to completely dissolve the aromatic polycarboxylic acid is notused, the volume of the reactor used can be small and the purificationof the aromatic polycarboxylic acid can be carried out efficiently.

Also, according to the present invention, it is not necessary to heat toa temperature required to completely dissolve the aromaticpolycarboxylic acid. Therefore, devices and utility for heating to ahigh temperature are not needed. Further, it is possible to avoidexcessive hydrogenation or decarbonylation, the elimination bydecomposition of carboxyl groups and the formation of polymerizationinhibitory substances and substances causing coloration which would beotherwise caused by heating to a high temperature. Hence, high purityaromatic polycarboxylic acid can be easily obtained.

Additionally, according to the present invention, it is possible topurify an aromatic polycarboxylic acid which is substantially impossibleto be purified by distillation because the self-decompositiontemperature thereof is lower than the boiling point thereof and which isdifficult to be purified by crystallization because the solubilitythereof in a solvent is low. Further, it is possible to obtain anaromatic polycarboxylic acid which can be used directly as such for thepolymerization to give a high molecular weight, colorless polyesterresin, etc.

EXAMPLES

The present invention will be described more concretely by way ofexamples and comparative examples. The present invention is, however,not limited to the examples.

In the present invention, OD₃₄₀ and OD₄₀₀ which are factors showing thedegree of containing coloring impurities are measured values obtained asfollows:

-   OD₃₄₀: 2 g of terephthalic acid are dissolved in 25 ml of 2N KOH and    the solution is charged in a 50 mm cell. Absorbance at 340 nm is    measured.-   OD₄₀₀: 1 g of naphthalenedicarboxylic acid is dissolved in 10 ml of    1N KOH and the solution is charged in a 10 mm cell. Absorbance at    400 nm is measured.

The values reflect the amount of coloring impurities and substancescausing coloration contained in terephthalic acid andnaphthalenedicarboxylic acid. The lower the value, the smaller is theamount of the coloring impurities.

Example 1

A crude terephthalic acid (150 g) containing 3,500 ppm of4-carboxybenzaldehyde (hereinafter referred to as 4CBA) and showingOD₃₄₀ of 1.0 and 600 g of water were charged in an autoclave equippedwith a stirrer. To the stirrer, two baskets each provided with holes forpassage of a terephthalic acid slurry were attached. In the baskets, 20g of coconut hull activated carbon supporting 0.5% of Pd were contained.After closing the autoclave, a hydrogen partial pressure of 0.2 MPa wasestablished therein. With stirring, the contents in the autoclave wereheated to 230° C. From the solubility of terephthalic acid in water at230° C., the amount of terephthalic acid dissolved in 600 g of water iscalculated as 39 g. Heating was stopped 2 h after the temperature of230° C. had been reached. After cooling to room temperature,terephthalic acid was recovered, rinsed with water at 90° C. and dried.The terephthalic acid thus obtained was found to contain 10 ppm of 4CBAand to show OD₃₄₀ of 0.1.

The terephthalic acid was polycondensed with ethylene glycol to obtain apolyester. Pellets of the thus formed polyester were transparent.

Example 2

A crude 2,6-naphthalenedicarboxylic acid (150 g) containing 2,600 ppm offormylnaphthoic acid and showing OD₄₀₀ of 1.0 and 600 g of water werecharged in an autoclave, similar to that used in Example 1, equippedwith a stirrer. To the stirrer, two baskets containing 20 g of the samecatalyst as used in Example 1 were attached. After hydrogen partialpressure of 0.2 MPa had been established, the temperature was increasedto 280° C. with stirring. From the solubility of2,6-naphthalenedicarboxylic acid in water at 280° C., the amount of2,6-naphthalenedicarboxylic acid dissolved in 600 g of water iscalculated as 36 g. Heating was stopped 2 h after the temperature of280° C. had been reached. After cooling to room temperature,2,6-naphthalenedicarboxylic acid was recovered, rinsed with water at 90°C. and dried. The 2,6-naphthalenedicarboxylic acid thus obtained wasfound to contain 10 ppm of formylnaphthoic acid and to show OD₄₀₀ of0.040.

The 2,6-naphthalenedicarboxylic acid was polycondensed with ethyleneglycol to obtain a polyester. Pellets of the thus formed polyester weretransparent.

Example 3

A crude terephthalic acid (150 g) containing 3,500 ppm of 4CBA andshowing OD₃₄₀ of 1.5 and 600 g of water were charged in an autoclaveequipped with a stirrer. To the stirrer, two baskets each provided withholes for passage of a terephthalic acid slurry were attached. In thebaskets, 20 g of coconut hull activated carbon supporting 0.5% by weightof Pd were contained. After closing the autoclave, nitrogen was fed sothat the pressure therein was increased to 2 MPa. Then the pressure wasreleased to atmospheric pressure. Such procedures were repeated fivetimes so that oxygen contained in the system was completely substituted.Then, with stirring, the contents in the autoclave were heated to 230°C. From the solubility of terephthalic acid in water at 230° C., theamount of terephthalic acid dissolved in 600 g of water is calculated as39 g. Heating was stopped 2 h after the temperature of 230° C. had beenreached. After cooling to room temperature, terephthalic acid wasrecovered, rinsed with water at 90° C. and dried.

The terephthalic acid thus obtained was found to contain 10 ppm of 4CBAand to show OD₃₄₀ of 0.16. The terephthalic acid was polycondensed withethylene glycol to obtain a polyester. Pellets of the thus formedpolyester were transparent.

Comparative Example 1

The procedures of Example 3 were repeated in the same manner asdescribed using the same apparatus and raw materials as those in Example3, except that oxygen in the system after charging of the raw materialswas not substituted with nitrogen. As a result, the product was found tocontain 11 ppm of 4CBA and to show OD₃₄₀ of 0.34. The terephthalic acidwas polycondensed with ethylene glycol to obtain a polyester. Pellets ofthe thus formed polyester were slightly colored.

Example 4

A crude 2,6-naphthalenedicarboxylic acid (80 g) containing 1,400 ppm offormylnaphthoic acid and showing OD₄₀₀ of 1.0 and 600 g of water werecharged in an autoclave, similar to that used in Example 3, equippedwith a stirrer. To the stirrer, two baskets containing 20 g of coconuthull activated carbon supporting 0.5% by weight of Pd were attached inthe same manner as in Example 3. After closing the autoclave, nitrogenwas fed so that the pressure therein was increased to 2 MPa. Then thepressure was released to atmospheric pressure. Such procedures wererepeated five times so that oxygen contained in the system wascompletely substituted. Thereafter, with stirring, the contents in theautoclave were heated to 280° C. From the solubility of2,6-naphthalenedicarboxylic acid in water at 280° C., the amount of2,6-naphthalenedicarboxylic acid dissolved in 600 g of water iscalculated as 36 g. Heating was stopped 2 h after the temperature of280° C. had been reached. After cooling to room temperature,2,6-naphthalenedicarboxylic acid was recovered, rinsed with water at 90°C. and dried. The 2,6-naphthalenedicarboxylic acid thus obtained wasfound to contain 50 ppm of formylnaphthoic acid and to show OD₄₀₀ of0.10. The 2,6-naphthalenedicarboxylic acid was polycondensed withethylene glycol to obtain a polyester. Pellets of the thus formedpolyester were transparent.

Comparative Example 2

The procedures of Example 4 were repeated in the same manner asdescribed using the same raw materials as those in Example 4, exceptthat the catalyst-containing baskets were not attached. As a result, theproduct was found to contain 1,350 ppm of 4CBA and to show OD₃₄₀ of 0.9.The terephthalic acid was polycondensed with ethylene glycol to obtain apolyester. Pellets of the thus formed polyester were colored.

Example 5

The same crude 2,6-naphthalenedicarboxylic acid (150 g) as used inExample 4 and 600 g of water were charged in an autoclave, similar tothat used in Example 3, equipped with a stirrer. To the stirrer, twobaskets containing 20 g of coconut hull activated carbon supporting 0.5%by weight of Pd were attached in the same manner as in Example 3. Afterclosing the autoclave, hydrogen was fed so that the pressure therein wasincreased to 2 MPa. Then the pressure was released to atmosphericpressure. Such procedures were repeated five times so that oxygencontained in the system was completely substituted. After hydrogenpartial pressure of 0.2 MPa had been established, the temperature wasincreased to 280° C. with stirring. From the solubility of2,6-naphthalenedicarboxylic acid in water at 280° C., the amount of2,6-naphthalenedicarboxylic acid dissolved in 600 g of water iscalculated as 36 g. Heating was stopped 2 h after the temperature of280° C. had been reached. After cooling to room temperature,2,6-naphthalenedicarboxylic acid was recovered, rinsed with water at 90°C. and dried. The 2,6-naphthalenedicarboxylic acid thus obtained wasfound to have a formylnaphthoic acid content below the detectable limitand to show OD₃₄₀ of 0.06. The 2,6-naphthalenedicarboxylic acid waspolycondensed with ethylene glycol to obtain a polyester. Pellets of thethus formed polyester were transparent.

INDUSTRIAL APPLICABILITY

According to the present invention, by converting a crude aromaticpolycarboxylic acid into a slurry in an aqueous medium and by bringingthe slurry into contact with a metal catalyst in the absence of oxygento carry out the hydrogenation or decarbonylation of the polymerizationinhibitory substances and substances causing coloration while preventingcatalyst components from contaminating crystals, the temperature of thepurification operation can be lowered. Therefore, side reactions can besuppressed and a product having such a quality as to permit direct usethereof as such for polymerization can be obtained with goodproductivity. Also, simplification of apparatus and energy saving may beattained.

Accordingly, in accordance with the method of the present invention,aromatic polycarboxylic acids which have been hitherto difficult to bepurified can be now purified with good efficiency in an extremelyindustrially advantageous manner.

1. A method of purifying aromatic polycarboxylic acid, comprising a stepof slurrying a crude aromatic polycarboxylic acid in an aqueous mediumand a step of bringing the slurry into contact with a metal catalyst inthe absence of oxygen while maintaining the aromatic polycarboxylic acidin a slurried state at such a temperature that aromatic polycarboxylicacid dissolved in the aqueous medium exists together with undissolvedaromatic polycarboxylic acid and while preventing catalyst componentsfrom contaminating crystals.
 2. The method according to claim 1, whereinsaid contact with the metal catalyst is performed in the presence ofhydrogen.
 3. The method according to claim 1, wherein the metal catalystcomprises a carrier and a Group 8 metal supported on the carrier.
 4. Themethod according to claim 1, wherein a basket in which the metalcatalyst is held is immersed into the slurry, so that catalystcomponents thereof are prevented from contaminating the crystals of thepurified aromatic polycarboxylic acid.
 5. The method according to claim1, wherein the aromatic polycarboxylic acid is terephthalic acid.
 6. Themethod according to claim 1, wherein the aromatic polycarboxylic acid is2,6-naphthalenedicarboxylic acid.
 7. The method according to claim 1,wherein the aromatic polycarboxylic acid is 4,4′-biphenyldicarboxylicacid.
 8. The method according to claim 2, wherein the metal catalystcomprises a carrier and a Group 8 metal supported on the carrier.
 9. Themethod according to claim 2, wherein a basket in which the metalcatalyst is held is immersed into the slurry, so that catalystcomponents thereof are prevented from contaminating the crystals of thepurified aromatic polycarboxylic acid.
 10. The method according to claim3, wherein a basket in which the metal catalyst is held is immersed intothe slurry, so that catalyst components thereof are prevented fromcontaminating the crystals of the purified aromatic polycarboxylic acid.11. The method according to claim 2, wherein the aromatic polycarboxylicacid is terephthalic acid.
 12. The method according to claim 2, whereinthe aromatic polycarboxylic acid is 2,6-naphthalenedicarboxylic acid.13. The method according to claim 2, wherein the aromatic polycarboxylicacid is 4,4′-biphenyldicarboxylic acid.
 14. The method according toclaim 8, wherein a basket in which the metal catalyst is held isimmersed into the slurry, so that catalyst components thereof areprevented from contaminating the crystals of the purified aromaticpolycarboxylic acid.